1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to accurately controlling the lapping of an air bearing surface of a row of recording heads where an ABS damascene process was used to define a feature in the recording head, such as a wrap around shield.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more magnetic recording heads (sometimes referred to as sliders) that include read elements and write elements. An actuator/suspension and holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to fly a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves the actuator/suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
The read element of a recording head typically includes a pair of shields with a magnetoresistance (MR) sensor formed between the shields. The MR sensor may be a Giant MR sensor, a Tunneling MR sensor, or another type of sensor. When a read operation is performed with the read element, the recording head is positioned over a selected track of the magnetic disk. Bit transitions in the track of the magnetic disk emit magnetic fields. When the MR sensor passes over a transition, the magnetic fields change the resistance of the MR sensor. The resistance of the MR sensor is measured by passing a sense current through the MR sensor, and measuring a resultant voltage across the MR sensor. The voltage measured across the MR sensor is used to generate a read signal which represents the bits written on the magnetic disk.
The write element of a recording head typically includes a pair of poles that are separated at the ABS by a write gap. The poles are connected to one another at a distal end from the ABS by a back gap. The write element also includes a coil that is wrapped about the poles and/or the back gap. When an electrical current is passed through the coil, a magnetic field is induced across the write gap which is used to write the bit transitions to the magnetic disk.
Magnetic disk drives have typically been longitudinal magnetic recording systems, wherein magnetic data is recorded as magnetic transitions formed longitudinally on a disk surface. The surface of the disk is magnetized in a direction along a track of data and then switched to the opposite direction, both directions being parallel with the surface of the disk and parallel with the direction of the data track.
Unfortunately, data density requirements are fast approaching the physical limits. Overall data density (or areal density) may be improved by improving linear density and/or track density. To improve linear density, bit sizes on a track need to be reduced which in turn requires decreasing the grain size of the magnetic medium. As this grain size shrinks, the magnetic field required to write a bit of data increases proportionally. The ability to produce a magnetic field strong enough to write a bit of data using conventional longitudinal write element technologies is reaching its physical limit.
One way to achieve higher density recordings is with perpendicular recording. In perpendicular recording systems, bits of data are recorded magnetically perpendicular to the plane of the surface of the disk. The magnetic disk may have a relatively high coercivity material at its surface and a relatively low coercivity material just beneath the surface. A write pole having a small cross section and very high magnetic flux emits a strong, concentrated magnetic field perpendicular to the surface of the disk. This magnetic field emitted from the write pole is sufficiently strong to overcome the high coercivity of the surface material and magnetize it in a direction perpendicular to its surface. The magnetic flux then flows through the magnetically soft underlayer (SUL) and returns to the surface of the disk at a location adjacent to a return pole of the write element. The return pole of the write element typically has a cross section that is much larger than that of the write pole so that the magnetic flux through the disk at the location of the return pole (as well as the resulting magnetic field between the disk and return pole) is sufficiently spread out to render the magnetic flux too weak to overcome the coercivity of the disk surface material. In this way, the magnetization imparted by the write pole is not erased by the return pole.
In perpendicular recording, the width of the write pole defines the track width on the magnetic disk, so the write pole is typically fabricated with a small pole tip in order to reduce the track widths as low as possible. As linear densities increase, wrap around shields may be formed proximate to the pole tip. The wrap around shield improves write field gradient and the angle of the field for improved writing. The sides of the wrap around shield suppress side writing. The back edge (i.e., the edge opposite the ABS) of the wrap around shield defines the throat height of the shield. The throat height should be of comparable dimensions with the gap width and the track width.
As write elements are fabricated smaller for higher density recording, smaller throat heights need to be achieved, such as less than 100 nanometers. A common method of fabricating a wrap around shield uses photolithographic processes to define the back edge of the wrap around shield. Unfortunately, present photolithographic processes have tolerances only down to about 30 nanometers. Thus, if the back edge of a wrap around shield needed to be defined at 60 nanometers, then the photolithographic processes may be off by as much as 50% in defining that back edge.
To solve this problem, an ABS damascene process may be used to define the wrap around shield instead of photolithographic processes. A further description of an ABS damascene process is provided in U.S. Patent Application Publication 2005/0180048. For an ABS damascene process, a sacrificial layer is fabricated with the photolithographic processes in place of the wrap around shield. The sacrificial layer may be Si, SiO2, Ta, W, etc. The size and back edge definition of the sacrificial layer do not have to be exactly defined with the photolithographic processes as would a wrap around shield. The sacrificial layer may be much larger than an actual wrap around shield, and the back edge may be define defined much further from the ABS than the actual throat height.
When the wafer level fabrication is complete, the wafer is parted into rows of recording heads. A row of recording heads is then lapped to define an ABS or close to defining the ABS. The lapping process also defines the stripe height of the MR sensors in the recording heads. The depth to which the row is initially lapped is controlled based on resistance measurements of lapping guides that are formed in the row. The lapping guides may be the MR sensors formed in the recording heads, or Electrical Lapping Guides (ELG) that are specially fabricated in the row for the purpose of controlling the lapping process.
After lapping is completed, the sacrificial layer in each recording head is selectively etched to define a void in each recording head where a wrap around shield will be formed. The sacrificial layer is etched to a depth which defines a back edge of a wrap around shield. The selective etching process has tight tolerances to define the back edge within about 5% of the target. With the voids formed in the recording heads, wrap around shield material is deposited on the ABS of the row of recording heads. The wrap around shield material fills the voids in the recording heads, which forms the wrap around shields having well-defined back edges.
Besides just filling the voids in the recording heads, the wrap around shield material covers the entire ABS of the row of recording heads. This wrap around shield material needs to be removed in a subsequent step. One problem with the ABS damascene process described is how to remove the wrap around shield material from the ABS of the row. The wrap around shield may be removed with an additional lapping process, but there is presently no effective way of determining when to stop the additional lapping process. If the row is lapped too long to remove the wrap around shield material, then the stripe heights of the MR sensors in the recording heads may be decreased beyond a desired level. At the same time, if the row is not lapped long enough, then the wrap around shield material will remain on the ABS of the row.
Another problem is that the wrap around shield material may be lapped unevenly from the ABS of the row. When lapping is performed on the row, the resistances of the lapping guides may be monitored to determine how evenly the row is being lapped. If the row is being lapped unevenly, then the lapping process is adjusts accordingly. However, when the wrap around shield material is deposited on the ABS of the row, the wrap around shield material shorts out each of the lapping guides. Thus, the lapping guides are not able to provide the traditional resistance values that can be used to control the lapping process.